In methods of forming images by electrophotography, electrostatic printing or the like, a developer charged electrostatically (hereinafter “toner”) flies to the surface of a photosensitive drum by electrostatic force which accords with potential differences on the drum surface, and develops electrostatic latent images formed on the drum surface. Hence, it is necessary and indispensable to control charge characteristics of the toner. Then, as a method for providing the toner with proper charge characteristics, a method is known in which a surface layer of a developer bearing member (hereinafter also “developing roller”) or the toner itself is incorporated with a charge control agent.
Conventionally, as negatively charging charge control agents, metal complexes of monoazo dyes, metal complexes of salicylic acid, alkylsalicylic acids or benzilic acid and so forth are used (PTL 1).
Recently, because of safety, concern about environments and a requirement for stabler charge characteristics, it is proposed to use as the charge control agent a resin having charge control function. In PTL 2, it is proposed that a resin having a sulfonic acid group is added to a surface layer of the developing roller to thereby make the toner improved in its uniformity of triboelectric charging and its running stability.
Meanwhile, in recent years, images to be reproduced are desired to be of much higher image quality, and faulty images have come into question, such that “fog” occurs where the toner comes into development at blank or white areas of images and that any streaky density non-uniformity occurs on images. What is a large factor of such a phenomenon is the occurrence of a toner having come charged to a polarity reverse to the desired charge polarity.
For the purpose of making a toner charged to a reverse polarity less occur or keeping the toner from being so charged, PTL 3 discloses therein an example in which a salicylic acid metal complex is used as a charge control agent, and PTL 4 discloses therein an example in which a polymeric compound composed of a monomer unit having a sulfonic acid group is used as a charge control agent. PTL 5 also discloses therein an example in which a polymeric compound to the polymeric backbone chain of which a salicylic acid unit is directly bonded is used in the toner.